1. Field of the Invention
The present invention relates to a molding die apparatus and a molding method thereof suitable for molding a resin board for use as an optical disc substrate such as in a compact disc (CD; a trademark), a digital versatile disc (DVD), a magneto-optical disc (MO) and the like, which requires high precision optical properties.
2. Description of the Related Art
As optical discs that can record and reproduce information by means of laser beam irradiation, there have been introduced the compact disc (CD: a trademark), the digital versatile disc (DVD), the magneto-optical disc (MO) and the like.
All of these types of optical discs have a recording portion for recording and reproducing information formed by a functional film such as a recording layer, a reflection layer and the like on a transparent resin substrate. In this case, for example, on one main surface of a disc-shaped transparent resin substrate having pits formed in an irregular pattern corresponding to a designated information signal, there are formed a recording film such as a phase-change film, a vertical magnetization film, and/or a reflection film of aluminum or the like by sputtering, and further a coating protection film is formed thereon.
Such optical disc provided as described above is rotated driven by a spindle motor mounted on a disc table, and reading of its information is enabled by means of an optical pick up apparatus which irradiates a laser beam from the other main surface of the transparent resin substrate and reads out information from its reflected light. Therefore, depending on the optical property of this transparent resin substrate, the reliability and a high speed processing of information are determined. Thereby, an optically homogeneous property is required therefor.
It is necessary for such optical disc substrates having the above-mentioned structure to be suitable for mass production and be fabricated under uniform dimensional precision. Therefore, as shown in FIG. 5, they are manufactured according to the following: melting a resin such as polymethylmethacrylate (PMMA), polycarbonate or the like at heating; injecting a molten resin from an injection molding machine 50 into a resin fill cavity provided within a disc substrate molding die apparatus 51 by a so-called injection molding method; and solidifying by cooling.
The above-mentioned disc substrate molding die apparatus 51 for molding the disc substrate is generally provided with a stationary die 52 and a moving die 53 for forming a cavity therebetween corresponding to a disc substrate. The stationary die member 52 is provided with a stamper for forming a plurality of pits with an irregular pattern that corresponds to a designated information signal on one of the main surfaces of the disc substrate. On the other hand, the moving die member 53 is provided with a read-out surface-forming member for mirror-finishing the other main surface of the disc substrate as an information read-out surface, and a punch for punching a spindle axis support hole in the center of a protruding portion of the optical disc.
When molding the resin substrate by such injection molding method, as molten resin is extruded from a nozzle into a cavity space formed between the die members, if the air in the cavity space is not fully exhausted and resides, it prevents filling of the resin. If filling of the resin is not performed smoothly, there occurs a problem that an optically is not performed smoothly, there occurs a problem that an optically non-homogeneous portion is generated on the molded resin which, if used as it is, would cause a read error, a write error or the like in the optical disc requiring essentially a stringent optical property and, due to deterioration in its bit transfer rate, birefringence and the like its optical property is thereby substantially affected.
Therefore, in order to smoothly exhaust the air in the cavity in the molding dies for molding these optical disc substrates, a countermeasure such as a so-called gas vent or the like is provided so as to exhaust the air and a resin gas emitted from the molten resin and residing in the cavity. However, because of a faster speed of injection of the resin, there arises such a condition in that the molten resin pushes the air out. Thereby, its exhaustion does not fully operate as intended, thereby preventing the filling of the resin and causing defects such as voids and the like trapped in the surface of the substrate due to residual air therein. Further, resin gas emitted from the molten resin filled in the cavity is solidified to clog the gas vent. Thereby, much time has to be spent in maintenance and cleaning the dies.
As to the conventional gas vent method, it is generally practiced that a vent groove is provided along the outer circumference of a product on a portion between parting surfaces of the stationary die and the moving die members and through which the gas is exhausted out of the cavity, or that as shown in FIG. 5, through a piping or a hose 54 which communicates with a vacuum exhausting apparatus 55 provided outside the molding machine, the gas is exhausted outside the cavity by operating the vacuum ON/OFF solenoid valves 57 and the like in response to a signal from a signal cable 56. However, because of a large air flow resistance through a long passage of piping to the vacuum exhausting apparatus 55, there occurs such a problem that its exhaustion rate becomes slow thereby failing to attain a sufficient effect of exhausting. Further, there occur such problems that because of the insufficient filling of the molten resin as well as the solidification of the resin gas therein described above, the optical property of the resultant optical disc is deteriorated, and that the vent tends to be clogged with resin debris.